In minimally invasive spinal surgery, pedicle screws comprising extended legs offer the possibility—especially in vertebral repositioning—to be able to do without various instruments and thus result in a significantly smaller incision in the skin. This allows to reduce the stressing of the patient during the operation and to accelerate the healing after the operation. In vertebral repositioning, tulip head pedicle screws comprising extended legs are screwed into the vertebral pedicles first; after this, an alignment rod is inserted into the tulip heads of several pedicle screws for mutually aligning the pedicle screws. If the pedicle screws have been placed in the vertebral pedicles at the desired positions and correctly aligned, the surgeon breaks off the extended legs of the pedicle screws or its tulip heads. In order to facilitate this procedure and allow for breaking the legs in a burr-free manner, pedicle screws comprising extended legs each comprise a predetermined breaking point between the screw head and the legs.
The extended legs increase the load on the predetermined breaking point as a result of deformation forces which are caused by tissue stresses and the manipulation on the part of the surgeon. This is why the position of the legs has to be secured in order to prevent the legs from breaking off too early.
To this end, locking rings or locking sleeves are known which can be slipped over the extended legs of a pedicle screw to stabilize the legs during inserting and aligning the pedicle screw.
A simple leg protection sleeve in the form a simple ring which can be slipped over the legs of a pedicle screw, is known from DE102011111403A1, for example. The inner radius of this leg protection sleeve corresponds to the outer radius of the tulip head of the pedicle screw and the inner circumferential surface of the leg protection sleeve is formed to be smooth. The disadvantage of this solution is that the position of the legs within the leg protection sleeve is not determinedly defined as the legs can move along the smooth inner circumferential surface of the leg protection sleeve in uncontrollable manner. Moreover, the accommodation depth of the legs in the leg protection sleeve is not defined either and the leg protection sleeve can get out of place along the axial direction of the legs, which is undesired.
In addition, the leg protection sleeve does not have any other function apart from stabilizing the legs; thus, the process of aligning several pedicle screws is performed in each case with one such leg protection sleeve by inserting a correction rod in the respective tulip heads of the pedicle screws. This has the effect that the correction rod is very close to the tissue of the patient and any possible deformation of the correction rod by the surgeon may lead to an unnecessary damage of the tissue.
A somewhat better defined way of positioning the legs in the locking ring is offered by a locking ring which is commonly known from the prior art, in particular by the locking ring known from US20130096635A1.
The commonly known locking ring is a half ring comprising two mounting openings or separate channels for the two legs of a pedicle screw. This locking ring can be slipped over the legs of a pedicle screw, too, but threading the legs into the respective mounting opening is time-consuming and difficult. Also with the commonly known locking ring, the accommodation depth of the legs in the leg protection sleeve is not defined and the locking ring may undesirably slip along the axial direction of the legs. Apart from the stabilization of the legs, the commonly known ring does not fulfil further functions and the previously described difficulty with inserting a correction rod in the tulip heads of the pedicle screws during their mutual alignment remains. What is more, the passage between the legs which is available for instrument access and defined by the legs is narrowed, as the mounting openings entirely enclose the legs. This makes it more difficult for the surgeon to position the pedicle screws, as the access channel between the legs is narrowed by the ring.
The locking ring according to US20130096635A1 also comprises two mounting openings for the two legs of a pedicle screw. In order to keep the legs in position and prevent an undesired movement of the legs, the mounting openings enclose the legs with very small clearance. This is why threading the legs into the respective mounting opening is here very time-consuming and difficult, too.
At its end facing away from the pedicle screw, the locking ring has a stopping piece in each of the mounting openings, which limits and defines the accommodation depth of the legs in the locking ring. In order to prevent an undesired slipping of the locking sleeve from the legs, each mounting opening additionally comprises a narrow groove in which a corresponding groove block of an extended leg is placed if the leg is inserted in the mounting opening. For releasing such anchoring and removing the locking ring from the legs of the pedicle screw, the locking ring has provided its outer side with an elastically deformable pressure arm adjacent to a groove, and the surgeon can apply an external pressure on said pressure arm whereby the latter moves into the groove, pushes the groove block out of the groove and thus separates the leg from the locking ring.
This locking ring admittedly offers a safe and defined positioning of the legs of a pedicle screw in the locking ring, but the design of the locking ring comprising several stopping pieces, narrow mounting openings, narrow grooves and pressure arms results in too many undercuts and blind areas where dirt or tissue particles arising during operation may accumulate. The cleaning of a locking ring having such a complex design is very time-consuming and difficult.
Apart from the stabilization of the legs, the locking ring known from US20130096635A1 does not fulfil any further functions and the problem of damaging the tissue during inserting a correction rod in the tulip heads of the pedicle screws during their mutual alignment remains also in this case. The inner diameter of the passage between the legs (which is available for instrument access and defined by the legs) is also reduced, as the mounting openings enclose the legs at least in part, and the positioning of the pedicle screws is more difficult for the surgeon.
In order to avoid the problems related to the reduction of the inner diameter of the instrument-accessible access between the legs, it is possible to equip the instrument itself (e.g. a screwdriver) with a locking ring, as is known for instance from U.S. Pat. No. 8,496,661B2. In this arrangement, the locking ring is formed in one piece with the screwdriver, so that the legs will move at the same time and corresponding to the rotation of the screwdriver. This avoids any shearing effect on the extended legs.
The disadvantage is that the instrument connected with the locking ring in one piece can only be used for those screws for which the locking ring is suitable. Here, the problem of damaging the tissue during inserting a correction rod in the tulip heads of the pedicle screws during their mutual alignment exists as well.